Aircraft are controlled by a throttle control lever, which adjusts a throttle valve in the aircraft engine, and a speed control lever, which adjusts the speed of rotation of the engine and the propeller. The speed control lever controls a propeller governor. The propeller governor in turn controls a propeller pitch control mechanism. Accordingly, the governor serves to operatively couple the speed control lever to the propeller pitch control mechanism. The pitch of the propeller determines the load on the engine. As the pitch increases, the load on the engine increases. Conversely, as the pitch decreases, the load on the engine decreases.
A disadvantage of this system is that the pilot must control both the throttle control lever and the speed control lever simultaneously. Obviously, the pilot may select less than optimum speed control settings for a given throttle setting. Excess wear and tear on the engine and poor fuel efficiency may result from these less than optimal settings.
The turbo charging of internal combustion engines is usually controlled through a waste gate. The waste gate is disposed in a by-pass duct that connects a turbine inlet directly with a turbine outlet. Exhaust gasses by-pass the turbine as they pass through the by-pass duct. The position of the waste gate determines the admission of exhaust gasses to the turbine. Thus, the waste gate functions in the same way as a valve. By increasing or decreasing the admission of exhaust gas to the turbine, it is possible to influence a compressor's output. The compressor is connected to the turbine through a turbocharger shaft. The charge pressure produced by the compressor is, therefore, determined by the position of the waste gate.
In many instances, but particularly in automotive applications, the waste gate is actuated by means of a diaphragm cell that comprises a membrane that is acted upon by gas pressure, a spring that acts against the pressure exerted by the gas, and an operating rod. The operating rod forms the connection between the diaphragm and the waste gate, so that the waste gate can be opened and closed. The air charge generated by the compressor is usually used as the pressure medium within the diaphragm cell. If the gas pressure in the diaphragm cell changes, then the diaphragm and the operating rod move to a position where the force exerted by the gas and the force exerted by the spring are in equilibrium. The spring is disposed in a chamber that is vented to the atmosphere. In this way, the waste gate may be moved into various positions as a function of the gas pressure. The gas pressure is usually adjusted by an electromagnetic timing valve. The greater the opening, the higher the gas pressure (and vice versa). The timing valve itself is controlled by the Engine Control Unit (ECU).
Although this method is effective for controlling automotive applications, it is extremely problematic for applications used on aircraft engines. Should the timing valve or its control system fail, the valve may be left either fully open or fully closed, depending on the type of valve involved. This may result in the waste gate being either fully opened or fully closed. This, in turn, may result in an abrupt drop in charge pressure that may result in a loss of power. Alternatively, this may result in an increase in charge pressure, with a corresponding risk of damage to the engine. Both situations are hazardous in aircraft engine applications. In principle, excess pressure can be dissipated through special “pop-off”, or alternatively, relief valves, although such valves are relatively costly.
In aircraft applications, hydraulic-mechanical control systems are normally used today in order to actuate the waste gate. In such cases, motor oil itself is usually used as the pressure medium, and this oil acts on a hydraulic actuating piston through a hydraulic-mechanical controller-logic system. The actuator piston is connected to the waste gate and thus adjusts it. However, the system is relatively costly. The relatively high weight of the system is also a disadvantage. In addition, there is no redundancy built into the system, i.e., there is no backup system that can perform system functions that may be lost in the event of a failure. A hydraulic-mechanical system is more stable than the previously described system using an electromagnetic timing valve, which controls a diaphragm cell. However, in the event of a system failure in a hydraulic-mechanical system, it cannot be excluded that under unfavourable conditions, charge pressure could tend towards an extreme value, and this eventuality is associated with the dangers discussed heretofore.